1. Technical Field
Embodiments of the present invention relate to a sheet discharging device to discharge a sheet having an image thereon on a sheet discharging tray, and an image forming apparatus incorporating the sheet discharging device.
2. Related Art
Known image forming apparatus generally include a sheet discharging unit. In the sheet discharging unit, a pair of sheet discharging rollers conveys a recording medium such as a paper sheet to be discharged onto a sheet discharging tray. The sheet discharging unit has a full state detection feeler and a full state detection sensor disposed downstream from the pair of sheet discharging rollers in a sheet conveying direction to detect a vertical paper loadable position of recording media discharged on the sheet discharging tray. Some techniques have been disclosed to change the paper loadable height of recording media in the full state according to a state of paper curl of the recording medium or media on the sheet discharging tray, so as to prevent the recording media on the sheet discharging tray from falling off the sheet discharging tray and/or disarrangement of the discharging order of the recording media.
Japanese Patent Application Publication No. JP 2003-137479-A discloses a configuration of a paper delivery device of an image forming apparatus. The paper delivery device includes a full state detecting filler to detect a full state of a sheet discharging tray. By connecting an auxiliary filler to a filler main body and selectively changing a connecting position, the full state detecting filler determines the full state according to a paper condition. However, the center area of a recording medium in a lateral direction is the only detection target of the full state detecting filler, this technique is not effective to a paper curl at both side edges while being effective to a paper curl at the leading edge and/or the trailing edge. Moreover, it is not realistic the setting of the full state detecting filler is changeable according to type and size of various recording media.
Japanese Patent Application Publication No. JP 2003-128339-A discloses an image forming apparatus having a configuration in which a first contact member and a second contact member are attached at the top of an arm of an actuator included in a full state detection feeler. This configuration can cause respective contact surfaces in the paper sheet discharging direction to contact from mutually different directions with the top surface of the paper sheet on a discharging tray. This configuration is designed so that the contact surface of the first contact member or the contact surface of the second contact member contacts the top of the paper sheet reliably even when a curled paper sheet is discharged on the discharging tray. This configuration is expected to prevent misdetection in which a full state is detected due to curled paper sheet even if a relatively small amount of paper sheets is discharged on the discharging tray, thereby improving accuracy in the full state detection of paper sheets. However, because of the same reasons as JP 2003-137479-A, the technique disclosed in JP 2003-128339-A is not effective to a recessed paper curl at both side edges. Further, since the technique of JP 2003-128339-A is employed to a movable discharging tray, when a compact image forming apparatus in which the height of a discharging tray is fixed uses this technique, the full state of paper sheets is detected before the amount of paper sheets reaching a target number of paper sheets with respect to the paper sheets having a small amount of curl.
Japanese Patent Application Publication No. JP 2010-275056-A discloses a configuration that provides a technique to employ an actuator including a full state detecting section and a rear-end curling detecting section, so as to detect a sheet paper full state and a paper curl state with a simple configuration. However, this technique is literally targeted to detect the rear-end curls, and therefore is not effective to the recessed paper curl at both side edges. Rather than that, a paper regulating sheet is disposed to contact a paper sheet to be discharged to a discharging tray so as to correct the curves at both side edges in the lateral direction of the paper sheet. However, it is likely that friction generated due to the contact of the paper sheet with the paper regulating sheet causes friction can cause a paper discharging problem. Therefore, another countermeasure is required to this inconvenience.
Consequently, it is clear that the above-described techniques cannot prevent paper stacking failures when the stack of paper sheets discharged on a discharging tray includes has a paper curl at both side edges. In addition, it is difficult to secure the target number of paper sheets that can be discharged on the discharging tray. It has become general that a height from a discharging port to a discharging stack surface is set as small as possible to reduce a distance between the discharging port and the discharging stack surface. However, it is likely that this configuration causes paper stacking failures in discharged paper sheets notably.
It is strongly demanded in these days that image forming apparatuses have a characteristic to be used in various installation environments. When an image forming apparatus is installed under high humidity conditions, paper sheets containing moisture can increase an amount or degree of paper curl after passing through a fixing unit in which heat and pressure are applied to the paper sheets. If the paper sheets are curled, specifically on at least either side thereof, the leading edge of a subsequent paper sheet hits the trailing edge of a curled proceeding sheet on the discharging tray to push the discharged paper sheet, which can cause paper stacking failure. In the image forming apparatus having a full state detecting mechanism at a downstream side from a pair of conveying rollers in a sheet conveying direction to detect a paper loadable height on a discharging tray, a printing operation is stopped before such sheet force-out occurs to prevent occurrence of paper stacking failure. However, if the full state detection mechanism is set to operate when a paper curl is detected, the full state is detected before reaching the target number of paper sheets even when the small amount of curled papers is discharged. This problem can be solved by increasing the capacity (the depth) of the discharging tray. However, due to a strong demand from the market to reduce the size of an image forming apparatus, it is not easy to achieve a solution to sufficiently solve both problems.